http: Only send connection:keep-alive if necessary

In cases where the Agent has maxSockets=Infinity, and
keepAlive=false, there's no case where we won't immediately close the
connection after the response is completed.

Since we're going to close it anyway, send a `connection:close` header
rather than a `connection:keep-alive` header.  Still send the
`connection:keep-alive` if the agent will actually reuse the socket,
however.

Closes #5838

lib/_http_client.js

@@ -107,9 +107,18 @@ function ClientRequest(options, cb) {
     var conn = self.agent.createConnection({ path: self.socketPath });
     self.onSocket(conn);
   } else if (self.agent) {
-    // If there is an agent we should default to Connection:keep-alive.
-    self._last = false;
-    self.shouldKeepAlive = true;
+    // If there is an agent we should default to Connection:keep-alive,
+    // but only if the Agent will actually reuse the connection!
+    // If it's not a keepAlive agent, and the maxSockets==Infinity, then
+    // there's never a case where this socket will actually be reused
+    var agent = self.agent;
+    if (!agent.keepAlive && !Number.isFinite(agent.maxSockets)) {
+      self._last = true;
+      self.shouldKeepAlive = false;
+    } else {
+      self._last = false;
+      self.shouldKeepAlive = true;
+    }
     self.agent.addRequest(self, options);
   } else {
     // No agent, default to Connection:close.

test/simple/test-http-raw-headers.js

@@ -34,13 +34,13 @@ http.createServer(function(req, res) {
     'x-BaR',
     'yoyoyo',
     'Connection',
-    'keep-alive'
+    'close'
   ];
   var expectHeaders = {
     host: 'localhost:' + common.PORT,
     'transfer-encoding': 'CHUNKED',
     'x-bar': 'yoyoyo',
-    connection: 'keep-alive'
+    connection: 'close'
   };
 
   var expectRawTrailers = [
@@ -77,7 +77,7 @@ http.createServer(function(req, res) {
     'Date',
     'Tue, 06 Aug 2013 01:31:54 GMT',
     'Connection',
-    'keep-alive',
+    'close',
     'Transfer-Encoding',
     'chunked'
   ];
@@ -96,13 +96,13 @@ http.createServer(function(req, res) {
       'Date',
       null,
       'Connection',
-      'keep-alive',
+      'close',
       'Transfer-Encoding',
       'chunked'
     ];
     var expectHeaders = {
       date: null,
-      connection: 'keep-alive',
+      connection: 'close',
       'transfer-encoding': 'chunked'
     };
     res.rawHeaders[1] = null;

test/simple/test-http-should-keep-alive.js

@@ -42,6 +42,7 @@ var SHOULD_KEEP_ALIVE = [
 ];
 var requests = 0;
 var responses = 0;
+http.globalAgent.maxSockets = 5;
 
 var server = net.createServer(function(socket) {
   socket.write(SERVER_RESPONSES[requests]);